Audiphone



Patented Dec. 29, 1942 AUDIPHONE Warren P. Mason, West Orange, N. 3.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application May 9, 1941, Serial No. 392,689

p 3 Claims. 'This invention relates to audiphones and the object of theinvention-is to'prevent an audiphone fromsinging when operating at highgain with the pick-up transmitter close to the receiver.

It has been suggested heretofore that for a very compact audiphone thepick-up transmitter. should be mounted together with a receiver in asingle casing which is held in position against the ear of the user. Forbinaural systems this arrangement would have the further'distinctadvantage that the microphones turn with the head of the user andthereby assist him in locating the direction of a sound source insomewhat the .same manner as normal ears.

In modern audiphone practice, however, an amplifier of either themicrophone or vacuum tube type is ordinarily used between thetransmitter and receiver and since the gain used at times may be as muchas 50 decibels special precautions must be taken to avoid singing due toacoustic or other coupling between the output and input circuits evenwhen the pick-up transmitter is mounted on the lapel or at some otherplace relatively remote from the receiver. Due to the tendency of thesehigh gain systems to sing, it has been necessary heretofore to foregothe advantages of mounting the pick-up transmitter at the ear and toplace it in some more stable operating position, such as on the coatlapel.

According to this invention the acoustic feedback from the receiver tothe pick-up transmitter is neutralized by the feedback from a second ordummy receiver having an acoustic terminating network simulating theimpedance of the users ear. In the preferred structure two basicallysimilar receivers are symmetrically disposed with respect to the pick-uptransmitter in a common case. One of these receivers is adapted to beconnected to the ear by a direct acoustic path in the usual manner andthe other receiver having the artificial termination is connected to theamplifier output with opposite polarity as compared with the receiverworking into the real ear.

The considerations involved in the design of an artificial ear are wellknown, as shown for example by the article A voice and ear for telephoneinstruments, by Inglis, Gray and Jenkins in the Bell System TechnicalJournal for April, 1932. Those skilled in the art can, therefore, makethe acoustic network simulate the ear impedance to any degree ofaccuracy required but end receiver bya simple structure such as anacoustic path of the same cross section as the ear canal terminated-by adiaphragm simulating the ear drum. 1

In the drawing,

Fig. 1 isan audiphone unit according to the invention and Fig. 2 shows abinaural audiphone using units of thectype shownin Fig. 1.

In Fig. 1 the casing ,I has a chamber 2 housing a pick-up transmitterand two other chambers 3 and 4 for the receivers 5 and 6, respectively.The vibratory elements I, 8 and 9 of the transmitter and receivers maybe of any known type but the well-known bi-morph crystal elements ofRochelle salt are particularly well suited to the purposes of thisinvention since they may be very light in weight and, unlike magnetictypes, they do not give rise to undesirable coupling effects. Thecrystal elements 8 and 9 of the receivers are connected to suitablediaphragms 20, 2| in the usual manner and in the case of the receiver 5the sound waves generated by the diaphragm 20 are transmitted through asuitable ear-piece 22 to the users ear. In the case of the dummyreceiver 6, however, the sound waves are transmittedthrough the acousticchamber 23 to the diaphragm 24.

The transmitter crystal unit I may, for example, be secured to thecasing at its center l0 and to the diaphragm II at its ends as shown.Sound waves impinging on the diaphragm through the holes 12 in thecasing will stress the crystal unit so as to generate between thecrystal electrodes potentials varying with the sounds in the well-knownmanner and these potentials will be impressed on the input of theamplifier l3 over the conductors l4, l5.

The crystal elements 8 and 9 of the receivers may be connected to theoutput of the amplifier in series or in parallel by means of conductorsl6, l1 and l8, l9 as shown, but in either case care must be taken topole the receivers in opposite senses with respect to the amplifieroutput. With the receiver 5 in place against the ear of the user thereceivers 5 and 6 will both radiate vibrations, but due to theconnections of opposite polarity the sound waves from the two receiverswill be of opposite phase and hence will substantially cancel each otherin their effects on the crystal unit 1 of the transmitter.

A single unit'of the type shown in Fig. 1 may be used as an ordinaryaudiphone but for best results a duplicate unit should be provided forthe other ear so as to take full advantage of the in most cases itwill'be sufllcient to load the secimproved binaural effect obtained withtransmit- 2 ters at the ear positions. In such a binaural set theunitswould ordinarily be mounted on a suitable head-band orother-support 28 as shown in Fig. 2. In cases where the gainrequirements are met by small amplifiers of the microphone type theseamplifiers may also be mounted in the unit housings I.

It is, of course, desirableto keep the wiring of the two units of abinaural set separated as far as possible on the person to minimize thetransfer of energy between the channels since this type of cross-talkimpairs the binaural effect. It will also be'understood that with anygiven structure according to the invention it may often be possible toimprove the acoustic balance emthe mastoid impedance in accordance withthe general theory as explainedjfor' example, in the article Anartificial. mastoid for audiphone measurementsf by Ms. S. Hawley irrtheBell Laboratories Recordfor November, 1939.

What is claimed is:

1. In an audiphone, a sound pick-up transmit-' ter, a first receiver, anamplifier connecting the transmitter to the receiver, a second receiverhaving an acoustic termination simulating the impedance of a human earconnected to the amplifier in opposite polarity with respect to thefirst receiver and a casing housing the transmitter and both of thereceivers.

2. In an audiphone, an amplifier having input and output circuits, asound pick-up transmitter connected to the input circuit, two receiverssymmetrically disposed with respect to the transmitter and connected inacoustically opposing relationship to the output circuit, and means forloading one'of the receivers to simulate the loading on the otherreceiver when. in contact with a human ear.

3. In a binaural audiphone including two sound amplifying channels eachcomprising a sound pick-up transmitter, a receiver, an amplifier con!necting the transmitter to the receiver and a housing for eachtransmitter and its associated receiver, and means for mounting thehousing on the head of a user with the receivers in operative relationwith the user's ears, other receiving means for each channel disposedwithin the housing and connected to the associated amplifier forproducing at the transmitter a sound field in phase opposition to thesound field produced by the receiver.

WARREN P. MASON.

